The disclosure relates generally to the field of electronic devices. More particularly, the disclosure relates to methods and systems directed to a vertical N-polar III-nitride based transistor.
Gallium nitride (GaN) is becoming the material of choice for power electronics to enable the roadmap of increasing power density by simultaneously enabling high-power conversion efficiency and reduced form factor. This is because the low switching losses of GaN enable high-frequency operation which reduces bulky passive components with negligible change in efficiency. Commercialization of GaN-on-Si materials for power electronics has led to the entry of GaN devices into the medium-power market since the performance-over-cost of even first-generation products looks very attractive compared to today's mature Si-based solutions. On the other hand, the high-power market still remains unaddressed by lateral GaN devices. The current and voltage demand for high-power conversion application makes the chip area in a lateral topology so large that it becomes difficult to manufacture. Vertical GaN devices would play a big role alongside silicon carbide (SiC) to address the high-power conversion needs.
Power conversion is ubiquitous in our everyday lives. It plays a role from charging our cell phone to powering our home. Power conversion could mean stepping up or stepping down from one voltage level to another (boost or buck) or a conversion from dc to an ac voltage (inverter) or from 1-phase to 3-phase (phase converter), or just isolating from the supply line (power factor correction). A switch can be regarded as the heart of any power conversion unit. An ideal switch is one which offers an infinite resistance to current in its OFF-state and zero resistance when in its ON-state. In solid state power electronics application a switch is realized by a transistor in its class D or higher operation. With advancement in solid state technologies the whole range of power electronics application can be addressed by solid state devices. According to a 2012 Presentation by Yole Development at CS-Europe (hosted by Compound Semiconductor), the range of power applications that can be addressed with GaN is shown in FIG. 1.
Si transistors have been providing the solutions for the entire range of voltages needed for power conversion ranging from 100 s of Watts to Megawatts with various devices like MOSFETs, IGBTs, SJTs, BJTs and thyristors. However the advent of wide bandgap (WBG) materials, and their rapid technological progress, promises enhanced performance beyond the Si roadmap. The higher critical electric field (Ec) due to the large bandgap of these materials makes them ideal for high-power electronics applications. Increasing operating voltages need higher Vbd and higher efficiencies need lower RON which is simultaneously best served by WBG materials.
GaN devices can be configured in a lateral or vertical configuration. In a typical lateral device, a thin layer of AlGaN is grown on top of the GaN channel to take advantage of the high mobility (˜2000 cm2 V−1 s−1) two-dimensional electron gas (2 DEG) formed at the AlGaN/GaN interface, which is used as the current carrying layer. The source, drain, and gate are fabricated on the same plane on the top of the typical lateral device. Electrons are modulated by the gate and flow from the source to the drain, where source-drain distance is primarily responsible for the blocking voltage in the off-state. However, for higher power (>10 kW) applications where higher breakdown voltages (>1.2 kV) are required, the lateral topology becomes increasingly unattractive both in cost and manufacturability due to the very large chip areas required by the breakdown voltages at the required current level (typically over 20 A).
Vertical topologies become more economical and viable for such a range of high power applications. A typical vertical device has a source and gate on the top and the drain on the bottom. One common example is a current aperture vertical electron transistor (CAVET). The current is controlled by the gate and the current flows through the bulk of the material into the drain. The horizontal high-mobility electron channel achieved by the AlGaN/GaN layer is used in conjunction with a thick GaN drift region in order to achieve low RON and a high breakdown voltage. Current blocking layers (CBL) are achieved by either p-type doping of the GaN layer or by implantation of a material like Mg or Al. In both cases, the devices require an aperture through which the current will flow. In existing technology, the CBLs are thus fashioned by applying a mask in the shape of the aperture, implanting the CBLs in the regions not covered by the mask, and regrowing the remainder of the GaN device. However, this regrowth process involves an interruption of a single crystal growth, which tends to produce imperfections at an interface where regrowth is performed. Alternatively doped (p-type) CBL can be formed by growth or regrowth. Let us call the structure prior to regrowth as the “Base structure” and the regrown structure as the “Regrown structure”. If the CBL, formed by doping, is a part of the base structure then the CBL in the aperture region needs to be etched and then the aperture region needs to be regrown in order to complete the device structure. If the aperture region is realized in the base structure then the CBL region is achieved by first etching the aperture layer in the designated CBL region and regrowing the CBL region with suitable doping. In either method regrowth is essential to fully fabricate the device.
The majority of GaN devices are produced with materials grown with Ga-polarity in the c-plane. Accordingly, the majority of current GaN device designs cannot achieve functions that are achievable by material properties that require materials grown with N-polarity.
Consequently, considering such limitations of previous technological approaches, it would be desirable to have a system and method for a producing a III-nitride vertical transistor with the above-mentioned functionality, but produced without a regrowth step.